FIG. 8 schematically shows a conventional ultra-high-pressure mercury lamp 200. This conventional ultra-high-pressure mercury lamp 200 is provided with a reflector 203 which has a surface that acts as a concave reflector formed on the inner surface thereof. A cylindrical protrusion, namely a neck part 203b, is formed on the outer surface of the reflector 203 at a position adjoining the bottom part of the concave reflector of the reflector 203. A luminous tube 202 of the ultra-high-pressure mercury lamp 200 has a cylindrical outer shape, and one end thereof runs from the bottom part of the reflector 203 through the neck part 203b, the luminous tube 202 being fixed together with a metal base 215 by means of cement 218 which is encapsulated between said luminous tube 202 and the inner wall of the neck part 203b. Front glass 219 is fitted into an opening 203a in the reflector 203 (see JP 2004-349194 A, for example).
As described above, the luminous tube 202 of the ultra-high-pressure mercury lamp 200 has a cylindrical outer shape, and one end thereof runs from the bottom part of the reflector 203 through the neck part 203b, the luminous tube 202 being fixed together with a metal base 215 by means of cement 218 which is encapsulated between said luminous tube 202 and the inner wall of the neck part 203b. In this case, the attachment of the luminous tube 202 and metal base 215 by means of the cement 218 is carried out in two stages. Firstly, when the luminous tube 202 is inserted into the reflector 203 after sealing with molybdenum foil at a quartz bulb, attachment by means of first cement 218 is performed (during alignment). In addition, when the metal base 215 is fixed, attachment by means of second cement 218 is performed (during fitting of the metal base).
Although the first and second cements 218 mentioned above are the same type of cement, when the second cement 218 is injected and dried in an oven in the metal base fitting process, the linear expansion coefficient is different because of variations in the uniformity etc. (particle size, distribution etc.), and the quartz bulb close to the boundary of the first and second cements 218 is subjected to shear stress, which leads to issues in that cracks are produced in the quartz bulb.
Moreover, it is believed that the quartz bulb close to the boundary of the first and second cements 218 is also subjected to thermal stress (shear stress) outside of the process for producing the ultra-high-pressure mercury lamp 200, when said lamp is illuminated/extinguished, for example.